Multi-carrier modulated signals are widely used in telecommunications. Today's dominating wireless physical layer waveform is cyclic-prefix (CP-) orthogonal frequency division multiplexing (OFDM). It is used in fourth generation (4G) cellular systems like LTE(-A), as well as in IEEE 802.11 standards. A drawback of CP-OFDM is its spectral property of high side lobe levels. As a consequence, CP-OFDM has to be operated in strict time-frequency alignment in order to avoid inter-carrier interference.
Schaich, F.; Wild, T.; Chen, Y.; “Waveform contenders for 5G—suitability for short packet and low latency transmissions,” in proceedings of IEEE Veh. Technol. Conf. Spring (VTC'14 Spring), May 2014, ([reference 1]) discloses three candidate multicarrier waveforms for the air interface of fifth generation (5G) networks. According to this publication, Universal Filtered Multi-Carrier (UFMC), which is also denoted as Universal Filtered Orthogonal Frequency Division Multiplexing, UF-OFDM, seems to be a promising 5G waveform candidate.
FIG. 1 depicts a block diagram of a conventional transmitter chain 1000 for UF-OFDM in an uplink (UL) configuration. B many UF-OFDM sub-modules 1010_1, 1010_2, . . . , 1010_B are provided, each of which receives e.g. QAM (quadrature amplitude modulation)—modulated symbols s1k, . . . , sn, wherein the index k represents a specific user, and each of which outputs a respective time domain transmit vector x1k, x2k, . . . , xBk obtained depending on said QAM-modulated symbols in a manner explained in detail below. The so obtained B many time domain transmit vectors x1k, . . . , xBk are superposed (i.e., added) by adder 1020, and the sum signal xk obtained at an output of the adder 1020 is up-converted, e.g. to a radio frequency (RF) range, by means of up-conversion unit 1030, whereby a UF-OFDM modulated RF signal rfo is obtained. Optionally, said up-conversion unit 1030 may also perform further well-known RF processing such as filtering, amplification, and the like.
A detector (not shown) may receive the UF-OFDM modulated RF signal rfo which may also comprise noise and/or interference caused by the RF channel/other users/transmitters in a per se known manner. After conversion to a baseband frequency range, the received signal vector may be processed as known in the art to improve the received signal quality.
Referring back to FIG. 1, according to the conventional UF-OFDM technique described in the above mentioned paper of Schaich et al., the time domain transmit vector xk for a particular multicarrier symbol of a user “k” is obtained as the superposition (cf. adder 1020) of sub-band-wise filtered components, with filter length L and FFT (Fast Fourier Transform) length N:xk=Σi=1BFikViksik  (equation 1),wherein xk is a [(N+L−1)×1] vector, i.e. a column vector having (N+L−1) many rows, wherein Fik is a [(N+L−1)×N] matrix, wherein Vik is a [N×ni] matrix, and wherein sik is a [ni×1] vector. For the sake of simplicity, a time index “m” is not considered in equation 1.
For each of the B many sub-bands, indexed i, ni many complex QAM symbols—gathered in sik—are transformed to time domain by an IDFT-matrix Vik. This is exemplarily depicted for the first sub-band (i=1) by IDFT spreader unit 1012_1. The IDFT-matrix Vik includes the relevant columns of an inverse Fourier matrix according to the respective sub-band position (index “i”) within the overall available frequency range. The matrix Fik is a Toeplitz matrix, composed of a filter impulse response of a filter performing the linear convolution for filtering the time domain signals obtained by the IDFT-matrix Vik, wherein said filter functionality implementing said matrix Fik, or matrix Fik, for the first sub-band (i=1), respectively, is represented by said filter unit 1014_1.
In other words, UF-OFDM sub-module 1010_1 comprises the IDFT spreader unit 1012_1 and the filter unit 1014_1. The further UF-OFDM sub-modules 1010_2, . . . , 1010_B comprise a similar structure with a respective IDFT spreader unit (implementing IDFT-matrix Vik) and a respective filter unit (implementing matrix Fik), wherein—as stated above—IDFT-matrix Vik includes the relevant columns of an inverse Fourier matrix according to the respective sub-band position “i” within the overall available frequency range, and wherein matrix Fik comprises a suitable filter impulse response for each sub-band i.
By now, no efficient solution for an apparatus and a method capable of providing multicarrier modulated signals of UF-OFDM type, has been provided.